The invention concerns method and apparatus of optically storing and retrieving digital data stored as light altering characteristics on an optical material and providing fast random access retrieval.
Optical memories of the type having large amounts of digital data stored by light modifying characteristics of a film or thin layer of material and accessed by optical addressing with minimal or no mechanical movement have been proposed but have not resulted in wide spread commercial application. The interest in such optical recording and retrieval technology is due to its faster retrieval of large amounts of data compared to that of existing electro-optical mechanisms such as optical discs, and magnetic storage such as tape and magnetic disc, all of which require relative motion of the storage medium.
For example, in the case of optical disc memories, it is necessary to spin the record and move a read head radially to retrieve the data, which is output in serial fashion. This type of data accessing not only requires the power consumption of the mechanical drives but also places more burden on buffer or other random access memory after retrieval in order to accommodate high speed data addressing and other data operations of modern computers. Solid state ROM and RAM can provide the relatively high access speeds that are sought, but the cost, size, and heat dissipation of such devices when expanded to relatively large data capacities limit their applications.
Examples of various other efforts to provide the relatively large capacity storage and fast access of an optical memory of the type that is the subject of this invention are disclosed in the patent literature such as U.S. Pat. No. 3,806,643 for PHOTOGRAPHIC RECORDS OF DIGITAL INFORMATION AND PLAYBACK SYSTEMS INCLUDING OPTICAL SCANNERS and U.S. Pat. No. 3,885,094 for OPTICAL SCANNER, both by James T. Russell; U.S. Pat. No. 3,898,005 for a HIGH DENSITY OPTICAL MEMORY MEANS EMPLOYING A MULTIPLE LENS ARRAY; U.S. Pat. No. 3,996,570 for OPTICAL MASS MEMORY; U.S. Pat. No. 3,656,120 for READ-ONLY MEMORY; U.S. Pat. No. 3,676,864 for OPTICAL MEMORY APPARATUS; U.S. Pat. No. 3,899,778 for MEANS EMPLOYING A MULTIPLE LENS ARRAY FOR READING FROM A HIGH DENSITY OPTICAL STORAGE; U.S. Pat. No. 3,765,749 for OPTICAL MEMORY STORAGE AND RETRIEVAL SYSTEM; and U.S. Pat. No. 4,663,738 for HIGH DENSITY BLOCK ORIENTED SOLID STATE OPTICAL MEMORIES. While some of these systems attempt to meet the above-mentioned objectives of the present invention, they fall short in one or more respects.
For example, some of the systems proposed above have lens or other optical structure not capable of the requisite resolution to retrieve useful data density. The optical resolution of the data image by these prior lens systems does not result in sufficient data density and data access rate to compete with other forms of memory. Although certain lens systems used in other fields such as microscope objectives are theoretically capable of the needed resolutions, such lens combinations are totally unsuited for reading data stored in closely spaced data fields. Another difficulty encountered with existing designs is the practical effect of temperature and other physical disturbances of the mechanical relationship between the data film or layer, the lens assemblies and the optical sensors that convert the optical data to electrical signals. For example, the thermal expansion effects of even moderate density optical memories of this type can cause severe misregistration between the optical data image and the read out sensors. Similar difficulties are encountered in the required registration between the recording process and the subsequent reading operations. Intervening misregistration of the high density optical components can cause significant data errors if not total loss of data.
A single reflection folded path optical memory device, such as disclosed in my U.S. Pat. No. 5,436,871, reduces somewhat the system dimension along the optical axis; the measurement that has been referred to as thickness or height of the optical memory device. But for very thin systems such as are required to fit inside a notebook computer, or even inside a still more compact PDA (Personal Digital Assistant), a new configuration is desired.
One function of the reflector in a folded path optical memory device is to perform the field, or collector, lens function of the system disclosed in application Ser. No. 08/920,847. The "collector" lens accepts an array of collimated beams, one from each data spot on a page of data, and directs each beam to, and focuses each beam on, the focal or target plane, i.e., the sensor array. It is similar to an astronomical telescope where the parabolic mirror accepts parallel ray bundles from an array of stars, and directs and focuses each star bundle on to the focal plane. If it is desired to reduce the thickness of the memory device, the focal length of the reflector must be reduced. But if it is desired to retain the same field of view such as for keeping the same data capacity, i.e., the same size data record, several problems arise.
First, the beams that are furthest off-axis will become astigmatic. This is a fundamental failing of a simple parabolic that is troublesome even in astronomical telescopes. Wide field telescopes require additional corrector elements.
Second, the angle that the off-axis beams make with the sensor plane may become so large that the image will be geometrically smeared, or may even approach the sensor from the rear; obviously an ineffective configuration. The solution for an astronomical instrument is to add an additional optical element, for example, a corrector plate some distance in front of the mirror. But in the folded path optical memory system, there is limited room for additional discrete elements, and the desired compression is much more severe than usually contemplated for astronomical instruments.
Accordingly, one object of this invention is to provide an optical mass memory having random accessibility in a relatively compact size, especially in height comparable to or even smaller than tape and compact disc storage mechanisms, and yet still serving data processing equipment in the same manner that solid state random access memories move data into and from the processor's data bus.